1. Field of the Invention
This invention relates to a radio tour guiding system in general and, in particular, to a tour guiding system having superior crosstalk and noise suppression capabilities.
2. Description of the Prior Art
There are presently several systems available on the market for guiding visitors through museums and the like by means of a radio transmitter/receiver system. One such system is sold by Serac, and is installed in the Metropolitan Museum of Art in New York.
Several prior art tour guides systems are also disclosed in the patent literature. Arber, U.S. Pat. No. 3,118,144; McIntosh, U.S. Pat. No. 3,078,348; Margolin, U.S. Pat. No. 2,942,066; and Abel, U.S. Pat. No. 3,474,451, all disclose lecture broadcasting systems of the sort well known to those of ordinary skill in the art. Arber was one of the first low power, multi-frequency communication systems developed that was suitable for museum lectures and the like. It suffers from the major deficiency that it uses antennas that are not directly closed loop, i.e. they depend upon a common ground for the return connection. It has been found that such systems suffer from re-radiation within steel buildings and the like and therefore often respond poorly to noise and crosstalk. McIntosh is of importance in that it teaches the use of a tuned closed loop broadcasting antenna coupled to a transmitter by means of an isolation transformer. Abel teaches a similar technique but is restricted to an FM communications system.
Additionally, different types of antenna configurations have been experimented with in order to produce improved propagation characteristics. The following patents to Mr. Justin L. Turner are typical of the foregoing developments: U.S. Pat. Nos. 3,549,818; 3,598,923; and 3,673,334.
One major problem that has been experienced by prior art systems is the interference with the information signal by crosstalk and noise originating from other loops. One solution to this problem is to produce an FM communications system such as disclosed by Abel. However, the Abel approach is relatively complex and sophisticated and requires that no difference side bands be produced in the receiver by the beating together of adjacent or alternate channels, or the difference side bands of adjacent or alternate channels.
In this fashion, no difference side bands are generated which have amplitude levels sufficient to produce interference in the audio information band and thereby distract the listener. Therefore, a solution was sought which could employ the relatively simple AM circuitry as known in the prior art and achieve sufficient crosstalk and noise suppression so that adjacent loops do not significantly interfere with one another. As a partial solution to this problem a novel compressor/expander system was developed to suppress the undesirable crosstalk within the loops. In general, compression/expansion techniques have found use in some other communications areas. The following prior art patents are typical of the use of the compressor/expander technique: Levine, U.S. Pat. No. 2,795,650; Lehan, U.S. Pat. No. 3,158,864; Miller, U.S. Pat. No. 3,471,648; Takahashi, et al., U.S. Pat. No. 3,757,354. Additionally, this technique has also been used to reduce tape and disc audio noise as described in the following U.S. Pat. Nos.: Ray Milton Dolby: 3,631,365 and 3,665,345. A good treatment of the Dolby B system can be found in December, 1973 issue of the IEEE Spectrum, pages 31 and 32. While expansion/compression techniques have found some limited utility in other areas of radio communications, this appears to be a first time that such a technique has been successfully used to reduce the noise associated with a closed loop, low power, multi-channel lecture broadcast system. The technique appears to be so successful that it is possible to place adjacent loops as close as 18 inches from one another without a significant amount of crosstalk. Prior to the use of the compression/expansion technique in the lecture broadcast system, it was not possible to eliminate crosstalk without separating the loops by a considerable distance. Additionally, the compression/expansion system developed to solve this problem appears to have special features not found in prior art compression/expansion systems.
To further reduce noise and drain on the receiver power source, a novel squelch means has been included in the Applicant's receiver. The squelch means reduces the power consumption of the receiver whenever an incoming signal is below a certain threshold value. The receiver may also be equipped with a gravity operated, mercury switch which will completely disconnect the power source of the receiver from the rest of the circuit whenever the receiver chasis is inverted.